Thursday, April 29, 2010

Why is Reducing Salt in Our Food Important?

Last week, the Institute of Medicine (IOM) called on the US Food and Drug Administration (FDA) to mandate a maximum level of sodium in food (IOM report). They did this because high sodium intake contributes to high blood pressure, which is a contributor to heart disease and stroke.

The average person in the US takes in more than double the recommended amount of sodium. And for most people, the salt shaker on the dinner table isn’t the main culprit. It’s processed foods, which are often very high in sodium, even for foods one wouldn’t expect to be “salty.” A graphic in last Sunday’s New York Times shows this nicely (Graphic: Funny, they don’t taste salty). Two Eggo brand waffles have over a quarter of the daily recommended sodium intake, as does a half cup of some brands of cottage cheese.

If sodium is such a problem and such a contributor to high blood pressure, shouldn’t we just identify the folks with high blood pressure and have them lower their sodium intake? This is in fact, the approach our health system has used for the past decade or so, and without much success. Rates of high blood pressure in the US have remained largely steady since the late 1990s (Ostchega, et al. 2008). Taking a broader approach to the issue, by lowering sodium intake across the population can have a larger and more sustained impact.

Why? The answer can be a bit detailed and has to do with the science of prevention. But it’s well-established that making small, healthy changes across an entire population can have a much bigger impact on health than trying to greatly lower the risk in a small percentage of “high-risk” people who already have a condition. This “population-approach” runs counter to the way much of the US health care system works, which focuses so much on one-on-one care for people who already have a disease or a condition, like high blood pressure. Yet, while it’s important to continue to treat people who get sick, establishing small population-wide changes can greatly reduce or delay the number of people who actually do go on to get sick.

The biggest obstacle with the population approach, though, is figuring out how to affect such small changes across an entire population. Though people often know a lot of the steps they should take to improve their health – say, be more active, eat more fruits and vegetables, or lower their salt intake – enticing most people into doing these things can be difficult.

The sodium recommendation from the IOM is one effective way. Processed foods make up about 70 percent of the American diet, and so changing their composition would impact the health of most Americans, and in such a way that they don’t even have to think about it.

In an analysis by colleagues (Cook, Cohan et al. 1995) at Harvard Medical School, researchers compared whether they could more effectively lower rates of heart disease and stroke by taking a healthy lifestyle population approach or by offering medical treatment to all individuals whose diastolic blood pressure exceeded 95 mm Hg. What they found was that the population-based approach resulted in fewer cases of heart disease and stroke than the high-risk treatment approach.

The Harvard study found that even better than either approach alone was the combination of the two approaches, where the entire population was ‘treated’ through the food supply, and high-risk individuals received medical attention. This is what would happen if the FDA took up the IOM recommendation. The government would be facilitating a population shift in sodium intake and physicians would continue treating those with high blood pressure. Similar results have been reviewed by UK based researchers (Frost, Law et al. 1991).

Australian researchers have applied a similar model to shifting BMI and such an approach could be applied to numerous other risk factors for cancer and other chronic diseases. In the Australian study, Wendy Brown and colleagues reported that a middle of the road approach may be best. If a high-risk approach is taken where individuals in the top 20 percent are targeted for a large intervention to reduce their BMI by three units (about a 20 pound loss for someone 5’10”), diabetes is reduced by 17 percent and high blood pressure by seven percent. By shifting the whole population’s BMI by one unit (a typical prevention approach and the equivalent of altering the population’s sodium intake without also treating those with high blood pressure), they estimated diabetes would be reduced by 13 percent and high blood pressure by 10 percent. But, if a middle of the road approach is used, where those in the top 50 percent of BMI are shifted two units, diabetes decreases by 23 percent and hypertension by 12 percent -- the best scenario for both outcomes (Brown, et al. 2007).

What becomes clear from all these studies and all these data is that a population-based approach when used alongside the usual high-risk approach can have a huge affect on the health of a nation. When it comes to high blood pressure, and the burden of diseases it's associated with, it’s time to look beyond the usual and try something new. Reducing sodium in processed foods may just be the missing ingredient we need.

It's true that some people are "salt-sensitive" and respond more strongly to changes in salt intake than the general population. Although there's no easy and quick way to determine who is salt-sensitivity and who is not, anyone known to be salt-sensitive should take particular care to keep salt/sodium intake in check. There is good evidence, though, that the health benefits of lowering salt/sodium intake across the population would reach beyond just those who are salt-sensitive and include the general population.